AN2521

Functional check

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controls the voltage at the COMP pin of the L6668, thus regulating the output current. In

case of a dead short, the current cannot be limited effectively by U5 because it will be

unpowered. Therefore, additional, efficient protection circuitry on the primary side will be

needed. In this board the voltage at the ISEN pin of the L6668 is sensed and if it exceeds

pin decreases to a level below the UVLO threshold. It will then attempt to restart, but without

success if the secondary short has not been removed. This provides a low frequency hiccup

working mode, limiting the current flowing on the secondary side and thus preventing the

power supply from overheating and failing.

Figure 22 shows the circuit behavior during short circuit. Observe that the L6668 stops

decreases its consumption, thus increasing the duration of the OFF time, and avoiding high

dissipation on the secondary side under short conditions. The soft start capacitor will also

the turn-on threshold is reached, the circuit attempts to restart but it will cease operation

within a few milliseconds, repeating the sequence just described. The restart attempt will be

repeated indefinitely until the short is removed.

Figure 23, instead, shows the sequence of operation in short circuit when the short is

removed. As the figure illustrates, a new start-up sequence takes place and the circuit

resumes normal operation after a soft-start cycle.

Thanks to the TSM1014 and the HV current source of the L6668, the fault protection

sequences described in Figure 22 and Figure 23 do not change significantly for any other

input voltage, above all not in the input voltage range of the board.

The protection described previously works correctly even in cases where the output short is

applied during standby or no load operations. The L6668 protects the circuit via the

sequences that has been described for the full load operation, and the circuit resumes

Figure 22.

EVAL6668-75 W short circuit at full

load & 230 Vac-50 Hz

Figure 23.

EVAL6668-75 W short circuit

removal at full load & 230 Vac-50 Hz

CH1: Q2 drain voltage

CH2: SOFT START voltage - pin #11

CH3: output voltage

CH1: Q2 drain voltage

CH2: SOFT START voltage - pin #11

CH3: output voltage